1. Field of the Invention
The invention relates to a method and a system for regulating the average electromagnetic torque of a polyphase rotating electrical machine equipped with stator and/or rotor windings. The invention also relates to storage media and a data structure for carrying out this method.
2. Description of the Prior Art
To be more precise, the invention relates to a method for regulating a machine in which the stator and/or rotor windings are supplied with a polyphase voltage and a polyphase current that are generated by an inverter, the inverter being formed by switches whose switching is controllable, this method including:
a control step of switching the switches as a function of an instantaneous torque set point, using an exact response control process to do this so that the instantaneous torque set point is reached as soon as the next regulation time, and
a step at each regulation time of calculating, from an average torque set point, the instantaneous torque set point to be applied so that the average of the instantaneous electromagnetic torque of the machine converges toward said average torque set point.
In the remainder of the description, the term “motor” will be used to denote a polyphase rotating electrical machine, and the term “torque” will be used to denote the electromagnetic torque of this kind of machine.
The above methods have a very wide regulation dynamic range since the instantaneous torque set point can be modified at each regulation time, and it is reached as soon as the next regulation time.
Such control methods are thus particularly useful for applications in which the torque set point changes abruptly. For example, these methods are used to control drive motors of a rolling mill.
However, when the instantaneous torque set point is obtained by simple equalization, for example by assimilating the average torque set point with the instantaneous torque set point, there is a difference between the value of the average torque set point and the average of the instantaneous torque between two successive regulation times. The average torque set point is therefore either never reached perfectly, or it is reached by forming the average of the instantaneous torque over a large number of regulation times, so that the method no longer has a wide regulation dynamic range.
It is therefore an object of the invention to overcome this drawback by providing a method for regulating the average electromagnetic torque which has a wide regulation dynamic range.
The invention therefore relates to a method for regulating the average electromagnetic torque as described above, which method includes a step of determining the value of the harmonics of the voltage and/or the current which are generated by the inverter, and the instantaneous torque set point is also established during the calculation step as a function of this value of the harmonics, so as to produce an instantaneous torque set point suitable for limiting the difference between the average of the instantaneous electromagnetic torque, between two successive regulation times, and said average torque set point.
It has been found that the difference between the average torque set point and the average of the instantaneous torque between two successive regulation times is due to the fact that the inverter cannot generate perfectly sinusoidal voltages or currents from a direct current voltage. In reality, the voltage and the current that are generated are resolved into a sinusoidal component at a fundamental frequency and into sinusoidal components with higher frequencies, corresponding to the harmonics with an order greater than or equal to two. The fundamental frequency sinusoidal component is simply referred to here as the fundamental, whereas the higher frequency sinusoidal components are referred to as harmonics.
The fundamental creates a constant torque Γm over a fundamental period. The harmonics generate an oscillating secondary torque with a higher frequency. The instantaneous torque Γs of the motor is the result of superposition of the torque Γm and the oscillating torque. The oscillating torque and the torque Γm are mutually independent. The known methods, which calculate an instantaneous torque set point only as a function of the average torque set point, do not therefore take the oscillating torque into account. Even if, for example, the instantaneous torque at each regulation time is strictly identical to the average torque set point, the average of the instantaneous torque Γs between two successive regulation times will therefore not be equal to this average torque set point, since the instantaneous torque varies between these two times owing to the oscillating torque. The oscillating torque is therefore responsible for the difference between the average torque set point and the average of the instantaneous torque. This difference increases commensurately when the amplitude of the oscillating torque is large. Since the oscillating torque is created by voltage and/or current harmonics, the value of this difference is therefore a function of the value of the harmonics.
The above method corrects the deficiency of the known methods by taking into account not only the average torque set point, but also the value of the current and/or voltage harmonics, for calculating the instantaneous torque set point.